1. Field of the Invention
The present invention is generally related optical imaging, and more particularly it is related to a catadioptric optical system with total internal reflection for high numerical aperture imaging; the catadioptric optical system may find industrial application in microscope objective systems or lithographic projection systems, among others.
2. Description of the Related Art
Imaging apparatuses, such as a microscope, a lithographic projection system, or even a telescope, use purely reflective (catoptric), purely refractive (dioptric), or a combination of reflective and refractive (catadioptric) optical elements to form an image of an object. A microscope uses an objective optical system to observe a sample, such as a biological tissue, a defect on a semiconductor wafer or a surface of material. A lithographic projection system uses a projection objective to project an image of a pattern on a reticle onto a planar image surface of a semiconductor substrate (wafer). In a telescope, an objective lens, larger in diameter than the pupil of a human eye, permits the collection of enough light to make visible distant point sources such as stars that otherwise may not be observed. To produce a good image, these instruments must collect enough light reflected from (or transmitted through) an object, separate the details in the image, magnify the image, and render the details visible to the human eye or resolvable by an optical detector.
The ability to resolve fine object details at a fixed object distance, regardless of whether the details correspond to physically close features (as in a microscope) or to features separated by a small angle (as in a telescope), is determined by the instrument's resolution. Resolution (R) of a microscope is given by Equation (1).
                    R        =                  0.61          ×                      λ            NA                                              (        1        )            
Where λ is the wavelength of the light used, NA is the numerical aperture of the microscope's object space, and 0.61 is derived from the Rayleigh criterion.
From Equation (1), therefore, the resolution R can be improved by decreasing the wavelength λ, or increasing the NA. In terms of decreasing the wavelength λ, the use of ultraviolet (UV), deep ultraviolet (DUV), X-ray, and electron beam radiation has been investigated extensively for high-resolution applications in microscopy and lithography. However, these applications are prohibitively expensive, and accordingly there is greater need for imaging using the visible spectrum (wavelengths between 400-700 nanometers approximately), as in the case of optical microscopes.
Therefore, the vast majority of optical microscopes have objectives designed to fulfill certain NA requirements. NA is determined by the instruments' ability to gather enough light to resolve fine object details. In terms of its ability to gather enough light, the NA of a microscope is defined by Equation (2).NA=No sin θm  (2)
where No is the refractive index of the medium in object space, θm is the angle formed between the marginal ray that comes from the object and the normal to the surface where the marginal ray impinges (hereinafter θm is referred to as the “marginal angle”).
From the perspective of Equation (2), therefore, in order to obtain a high NA value, either the angle θm of the marginal ray or the refractive index No of the medium in object space need to be large. As it is generally known to persons having ordinary skill in the art, the medium in the object space of a microscope can be air or an immersion fluid. When air (No=1) is used in the object space, the maximum value of NA cannot be greater than unity, but when the object space is filled with a fluid of index larger than 1 (No>1) a NA larger than 1 can be achieved. Incidentally, most conventional optical microscopes use objectives with NA values in the approximate range of 0.08 to 1.30, with the proviso that NA values greater than 0.95 can typically be achieved only by using an immersion fluid in the object space. Accordingly, to further increase the NA value, regardless of the medium in the object space, the angle θm of the marginal ray needs to be increased. However, this requires significantly complicated optical arrangements for correcting aberrations.
Specifically, many conventional optical designs for high NA values use catadioptric optical elements to minimize optical aberrations. See, for example, U.S. Pat. No. 5,650,877, international publication number WO2008/101676 (herein “WO2008/101676”), and the article “A New Series of Microscope Objective: I. Catadioptric Newtonian Systems,” JOSA 39, No. 9, 719-723 (1949), by Grey et al. (herein “Grey”).
U.S. Pat. No. 5,650,877 discloses a lithographic reduction system in which a catadioptric optical element having specially configured front and back faces projects a reduced image of a reticle onto a substrate. The back face of the optical element has a central aperture surrounded by a concave reflective surface. The front face has a partially reflective surface that transmits a portion of the light beam toward the concave reflecting surface and reflects a portion of the remaining light beam returned by the concave reflective surface on a converging path through the central aperture. The substrate is aligned with the aperture, and is therefore exposed with high-resolution.
WO2008/101676 discloses a lithographic projection system in which a catadioptric optical element made of a high-index transparent material has a first surface on an object-side of the element and a second surface opposite to the first surface. The second surface has a transmissive portion in a central region around the optical axis and a concave reflective portion in a zone around the transmissive portion. The first surface has a transmissive zone to transmit radiation coming from the object surface towards the second surface and oriented relative to the second surface such that at least a portion of radiation reflected by the reflective portion of the second surface is totally reflected by the transmissive portion of the first surface towards the transmissive portion of the second surface.
Grey discloses a microscope objective with a last solid lens made of fluorite or quartz-fluorite, where both object-side and image-side surfaces of the lens contain reflective coating on certain regions thereof to achieve NA values greater than 0.95 at 220 to 540 nanometer wavelengths purportedly with negligible aberrations.
A feature common to each of the above-discussed background references is the last optical element which a catadioptric optical element (COE) in which a central obscuration blocks a portion of the light from passing through the central region thereof. Generally, the obscuration ratio—which characterizes the fraction of blocked illumination—is defined by the following equation (3):
                    Obscuration        =                              sin            ⁢                                                  ⁢                          θ              l                                            sin            ⁢                                                  ⁢                          θ              m                                                          (        3        )            
where θ1 is the lowest angle to achieve the required obscuration ratio (hereafter θ1 will be referred to as the “lowest obscuration angle”), and θm is the marginal angle, as defined in Equation (2). Accordingly, a central obscuration larger than a certain threshold (e.g., 25%) can cause significant degradation in image contrast and loss of light intensity, which in turn degrades the quality of a resultant image.
According to U.S. Pat. No. 5,650,877, the central obscuration may be limited by controlling the size of the illumination beam to block no more than 15 percent of the projected image. However, although relatively low obscuration may be obtained by controlling the size of the illumination beam, substantial energy loss is caused by this technique.
On the other hand, in the catadioptric optical element disclosed by WO2008/101676 total internal reflection (TIR) is used to minimize obscuration while achieving a desired level of NA. FIG. 1 illustrates a concept of the catadioptric optical element disclosed by WO2008/101676.
The left side of FIG. 1 illustrates a side view of a catadioptric lens 10, which has a first surface 11 and a second surface 12 opposite to each other. The first surface 11 is generally concave when seen from the side of the second surface 12, and the second surface 12 is substantially planar (flat). A plane view of the substantially planar second surface 12 is illustrated on the right side of FIG. 1. The first surface 11 has a transmissive portion in a central region around the optical axis AX and a concave reflective portion in a region around the transmissive portion. That is, the transmissive portion and the concave reflective portion are concentric to each other. The second surface 12 is generally transparent and has a total internal reflection (TIR) region 16 and transmissive region 17, which are concentric to each other and also centered on the optical axis AX. Light rays illuminating an object O passes through the transmissive portion of the first surface 11 and impinges first on the second surface 12. More specifically, light rays R2 and R3 having angles of incidence between the critical angle θc and the marginal angle θm undergo total internal reflection on the TIR region 16 of the second surface 12, and are therefore reflected towards the reflective portion of the first surface 11. In turn, the reflective portion of the first surface 11 reflects these rays forward towards the second surface 12 as light rays R2′ and R3′. This time, since the incident angles of rays R2′ and R3′ are less than the critical angle θc, the rays R2′ and R3′ are transmitted through the TIR region 16 of the second surface 12.
On the other hand, light rays R1 propagating through the transmissive region of the first surface 11 and impinging on the transmissive region 17 of the second surface 12, at an incident angle less than the critical angle θc (e.g., incident at the minimum obscuration angle θ1), cannot be reflected by the second surface 12, but instead these rays are refracted as a light rays R1′. The refracted rays R1′ may be scattered or blocked by a central obscuration or field stop aperture; thus, the light rays R1 with an incident angle θ1 less than the critical angle θc do not contribute to image formation. Moreover, the transmissive region 17 immediately around the optical axis AX is obscured because the object O itself blocks light incident normal to the object. Therefore, light rays impinging on the transmissive region 17 at incident angles small than the critical angle θc may degrade image contrast and cause loss of light intensity.
Furthermore, since a catadioptric optical element includes the above-described curved reflective surfaces, other problems in terms of chromatic aberration, Petzval curvature and alignment arise.
Correcting chromatic aberration, in particular, across the visible spectrum of wavelengths is particularly challenging. As it is known to persons having ordinary skill in the art, a microscope can be thought of as a positive lens. In that sense, the power of the positive lens produces what is known as “undercorrected” axial chromatic aberration. To compensate for it, overcorrected axial chromatic aberration is intentionally generated by adding specially designed optical elements within the microscope's optical system.
Image field curvature is another imaging aspect to be considered. Specifically, since an image of a sample is generally captured by a sensor, such as CCD (charged coupled device) or CMOS (complementary metal oxide semiconductor) sensor, which has a flat surface, a flat image is required at the plane where the sensor is located. Generally, however, since a microscope can be regarded as a positive lens, the power of the positive lens generates an image having an inward-curving field. The curvature of the resulting image is known as the Petzval curvature. To compensate for inward Petzval curvature, an outward-curving field is intentionally generated by adding specially designed optical elements within the microscope's optical system. Specifically, using a concave mirror has been known to be an effective method for compensating the inward Petzval curvature. It is clear, therefore, that correction of aberrations can considerably increase the number of lens elements that ultimately form the objective optical system of a microscope. This significant increase in the number of optical elements often results in a tight-fit, difficult to align, and oversized objective system.
Accordingly, there is a need for objective optical systems that can provide minimum obscuration, correction of chromatic aberration and Petzval curvature, and allow for appropriate alignment without undue difficulty.